This invention relates generally to integrated circuit processes and fabrication, and more particularly, an integrated circuit and method for having improved electrical conductivity between copper and a conductive surface.
The demand for progressively smaller, less expensive, and more powerful electronic products, in turn, fuels the need for smaller geometry integrated circuits (ICs), and large substrates. It also creates a demand for a denser packaging of circuits onto IC substrates. The desire for smaller geometry IC circuits requires that the interconnections between components and dielectric layers be as small as possible. Therefore, research continues into reducing the width of via interconnects and connecting lines. The conductivity of the interconnects is reduced as the surface area of the interconnect is reduced, and the resulting increase in interconnect resistivity has become an obstacle in IC design. Conductors having high resistivity create conduction paths with high impedance and large propagation delays. These problems result in unreliable signal timing, unreliable voltage levels, and lengthy signal delays between components in the IC. Propagation discontinuities also result from intersecting conduction surfaces that are poorly connected, or from the joining of conductors having highly different impedance characteristics.
There is a need for interconnects and vias to have both low resistivity, and the ability to withstand volatile process environments. Aluminum and tungsten metals are often used in the production of integrated circuits for making interconnections or vias between electrically active areas. These metals are popular because they are easy to use in a production environment, unlike copper which requires special handling.
Copper (Cu) is a natural choice to replace aluminum in the effort to reduce the size of lines and vias in an electrical circuit. The conductivity of copper is approximately twice that of aluminum and over three times that of tungsten. As a result, the same current can be carried through a copper line having half the width of an aluminum line.
The electromigration characteristics of copper are also much superior to those of aluminum. Aluminum is approximately ten times more susceptible than copper to degradation and breakage through electromigration. As a result, a copper line, even one having a much smaller cross-section than an aluminum line, is better able to maintain electrical integrity.
There have been problems associated with the use of copper, however, in IC processing. Copper pollutes many of the materials used in IC processes and, therefore, care must be taken to keep copper from migrating. The migration of copper into silicon semiconductor regions is especially harmful. The conduction characteristics of the semiconductor regions are a consideration in the design of a transistors. Typically, the fabrication process is carefully controlled to produce semiconductor regions in accordance with the design. Elements of copper migrating into these semiconductor regions can dramatically alter the conduction characteristics of associated transistors.
Various means have been suggested to deal with the problem of copper diffusion into integrated circuit material. Several materials, especially metallic ones, have been suggested for use as barriers to prevent the copper diffusion process. Tungsten, molybdenum, and titanium nitride (TiN) have also been suggested for use as copper diffusion barriers. However, the adhesion of copper to these diffusion barrier materials has been an IC process problem, and the electrical conductivity of such materials is an issue in building IC interconnects.
Copper cannot be deposited onto substrates, or into vias, using conventional metal deposition processes, such as sputtering, when the geometries of the selected IC features are small. That is, new deposition processes have been developed for use with copper, instead of aluminum, in the lines and interconnects of an IC interlevel dielectric. It is impractical to sputter metal, either aluminum or copper, to fill small diameter vias, since the gap filling capability is poor. To deposit copper, a chemical vapor deposition (CVD) technique has been developed in the industry.
In a typical CVD process, copper is combined with a ligand, or organic compound, to make the copper volatile. That is, copper becomes an element in a compound that is vaporized into a gas. Selected surfaces of an integrated circuit, such as diffusion barrier material, are exposed to the copper gas in an elevated temperature environment. When the copper gas compound decomposes, copper is left behind on the selected surface. Several copper gas compounds are available for use with the CVD process. It is generally accepted that the configuration of the copper gas compound, at least partially, affects the ability the copper residue to adhere itself to the selected surface.
A co-pending application, Ser. No. 08/717,267, filed Sep. 20, 1996, entitled, "Oxidized Diffusion Barrier Surface for the Adherence of Copper and Method for Same", invented by Tue Nguyen, Lawrence J. Charneski, and Lynn R. Allen, Attorney Docket No. SMT 123, which is assigned to the same Assignees as the instant patent, discloses a method for oxidizing the diffusion barrier surface to improve the adherence of copper to a diffusion barrier. In low speed electrical circuits the resistance offered by a thin level of oxide is unnoticeable. However, in higher speed applications even a small amount of resistance can reduce the propagation time of electrons across an oxide layer. The primary purpose of this, above mentioned, patent application is to improve the ability of copper to remain deposited on a surface, not on improving the conductivity between copper and another surface.
Another co-pending application, Ser. No. 08/717,315, filed Sep. 20, 1996, entitled, "Copper Adhered to a Diffusion Barrier Surface and Method for Same", invented by Lawrence J. Charneski and Tue Nguyen, Attorney Docket No. SMT 243, which is assigned to the same Assignees as the instant patent, discloses a method for using a variety of reactive gas species to improve copper adhesion without forming an oxide layer over the diffusion barrier. However, the focus of this patent is to improve copper adhesion, not to improve the conductivity of copper deposited on a surface. In addition, the method of the above patent is generally only applicable to diffusion barrier material.
It would be advantageous to employ a method of improving the electrical conductivity of CVD copper with a conductive surface.
It would be advantageous if a method were employed for preparing a conductive surface, in advance of CVD copper deposition, to improve the conductivity between copper and the conductive surface.
Further, it would be advantageous if the conductivity improving process did not degrade the adhesion between the deposited copper and the conductive surface. It would also be advantageous if the process did not disrupt silicon bonds and structures in adjoining IC substrates.
Accordingly, a method of enhancing the interface conductivity of copper applied to selected integrated circuit surfaces is provided. The selected copper-receiving surfaces are predominantly conductive surfaces applied to selected regions of the IC. The method comprises the steps of: exposing each selected copper-receiving surface to a low energy source of ions; removing contaminants from each surface of the copper-receiving surface in response to the ion exposure; and applying CVD copper on each copper-receiving surface exposed above, whereby the ion exposure promotes electrical conductivity between the deposited copper and the copper-receiving surface.
Preferably, the source ions are generated from an inert gas, whereby chemical reactions between the ions and the copper-receiving surface are minimized. The inert gas is selected from the group consisting of Ar, He, Me, Kr, H.sub.2, N.sub.2, and Xe. In one preferred embodiment of the invention, the source ions have an energy level of generally less than 150 eV, whereby the low energy of the ions minimizes the penetration of ions into the exposed copper-receiving surface.
The copper-receiving surface is selected from the group consisting of Cu, Ti, W, and Al, whereby the cleaning of the copper-receiving surface promotes adhesion and conductivity between the copper-receiving surface and the deposited copper. Alternately, the copper-receiving surface is selected from the group consisting of TiN, TiON, TiSiN, TaSiN, TaN, TiW, TiWN, No, and WN, whereby copper is adhered to a copper-receiving surface which acts as a conductive diffusion barrier between the deposited copper and regions of the IC underlying the copper-receiving surface.
In an alternative embodiment of the invention, a metal alternative to copper is applied to selected integrated circuit surfaces, and the CVD metal alternative is selected from the group consisting of Al, W, and Ti.
An adherent copper conductor interface in an integrated circuit interlevel dielectric is also provided. The adherent copper conductor interface comprises a first surface of IC material including a semiconductor connection area having a connection area surface, and a second surface of IC material overlying the first surface. The copper interface also comprises a via, having vertical walls, between the second surface and the connection area. The copper conductor interface further comprises a diffusion barrier layer overlying the via walls and the connection area, with the surface of the diffusion barrier being exposed to a low energy source of ions from an inert gas to clean contaminants from the diffusion barrier surface. Finally, the copper interface comprises a copper stud adhered to the diffusion barrier surface to electrically interface the connection area to the second surface, whereby the ion exposure of the diffusion barrier surface promotes electrical conductivity between the conduction area and the second surface.
In one embodiment of the invention, the copper-receiving surface is exposed to ions of an inert gas and oxygen, and includes a further step of stopping the exposure of the copper-receiving surface to the ion exposure before an oxide layer is formed that exceeds approximately 30 angstroms (.ANG.), whereby ions of the inert gas are used to clean the copper-receiving surface to improve electrical conductivity, and ions of the oxygen are used to form a thin oxide layer on the copper-receiving surface which improves adhesion of the copper-receiving surface to copper.
Another adherent copper conductor interface in an integrated circuit interlevel dielectric is also provided. The copper conductor interface comprises a first surface of IC material including a metallic connection area having a connection area surface, and a second surface of IC material overlying the first surface. The copper conductor interface comprises a via, having vertical walls, between the second surface and the conduction area surface. The copper conductor interface further comprises a diffusion barrier layer having a surface overlying the via walls. The diffusion barrier surface and the connection surface selected region are exposed to a low energy source of ions from the inert gas to clean contaminants from the diffusion barrier surface and the connection surface selected region. Finally, the copper conductor interface comprises a copper stud adhered to the diffusion barrier layer to electrically interface the connection area to the second surface, whereby the ion exposure of the diffusion barrier surface and the connection area selected region promotes electrical conductivity with copper.